A transmitter, such as a transmitter used by a 4G LTE Macro base station, often includes quadrature modulators and variable gain amplifiers. The transmitter typically up-converts quadrature baseband signals using two sinusoidal local oscillator (LO) signals that have the same frequency, but are out of phase with each other by 90 degrees. The transmitter does this by modulating one channel of data on a cosinusoidal carrier (the in-band or I data) and modulating the other channel of data on a sinusoidal carrier (the quadrature or Q data), with both carriers set to the same frequency. The two modulated carrier signals are then added together to form a composite signal that is transmitted via an antenna. Modern receivers have the ability to separate signals on these quadrature carriers, and to separate the I data from the Q data.
This type of transmitter is called a single sideband transmitter, because the image sideband is suppressed by mixing the quadrature baseband signals and quadrature LO signals in this fashion. In addition, by using these mathematical techniques rather than filtering, two channels of data can be transmitted within the same bandwidth, thus doubling spectral efficiency.
The composite signal generated by the transmitter often includes a desired sideband and an undesired image sideband due to imperfections in the modulation process. To reduce the effects of the image sideband (i.e., to have a higher image rejection), it is desirable to generate the quadrature LO signals so that they have good amplitude balance (i.e., they have approximately the same amplitude) and a good phase balance (i.e., a phase separation of approximately 90 degrees). This, however, is not always an easy task, especially for a wideband system, where a broad range of LO frequencies, such as with 4G LTE, is being supported.
Accordingly, it would be advantageous to have improved quadrature signal generation by generating respective LO signals with good amplitude and phase balance.